Pneumatic actuator system

ABSTRACT

A pneumatic actuator system including one or more piston-cylinder type actuators ( 14 ) intended for crust breaking operations at electrolytic alumina reduction baths, each actuator ( 14 ) having a working piston ( 21 ), and a piston rod ( 22 ) carrying a crust breaking working implement ( 17 ), and a control circuit including a directional valve ( 24 ) is arranged to operate the actuator piston ( 21 ) in alternative directions, wherein the control circuit comprises air feed flow restrictions ( 26, 27 ), end position sensors ( 28, 29 ) and air feed shut-off valves ( 30, 31 ) for minimising the pressure air volume needed for accomplishing complete working strokes of the actuator piston ( 21 ) at varying crust layer thickness.

[0001] This invention relates to a pneumatic actuator system includingone or more piston-cylinder type actuators, each having a working pistonwith a load engaging piston rod. The system further comprises a controlcircuit with a directional valve for directing pressure air toalternative sides of the working piston of each actuator foraccomplishing movement of the working piston in alternative directions,and flow restrictions for restricting the air feed flow to the actualdriving side of the working piston.

[0002] Actuator systems of this kind are used in the aluminium producingindustry, in particular for crust breaking operations in electrolyticalumina reduction pots. Aluminium producing plants are usually bigoperations having a great number of electrolytic baths for reduction ofaluminium oxide into metallic aluminium. For repeatedly breaking thecrust layers inevitably formed on top of the electrolytic baths andthereby enabling supply of alumina, i.e. pulverized aluminium oxide intothe baths, there are used a great number of big-size pneumaticactuators.

[0003] A problem inherent in this type of operations is that the crustlayers to be broken may vary in thickness from zero to a very massivecrust body, and to be able to deal with the thicker crust layers theactuators have to be big and powerful. For a big aluminium producingplant this creates a demand for a huge pressure air supply capacity,because driving the working piston of each actuator in reciprocatingcycles requires a large amount of pressure air. This causes substantialcosts, and there is a serious need in this type of industry to reducethe overall pressure air consumption and to bring down these costs

[0004] Previously, a solution to this problem has been suggested whichmeans that the current driving side of the actuator working piston isfed with pressure air via a flow restriction, whereas the oppositeidling side of the working piston is vented through a substantiallyunrestricted outlet. This means that the pressure on the driving side ofthe working piston is quite low as long as the resistance to the pistonmovement is low, but increases automatically all the way up to themaximum pressure available in case the resistance to piston movementbecomes higher.

[0005] In the above described field of use for pneumatic actuators, thecrust layers are very thin and result in very low piston loads in morethan 90% of all crust breaking cycles. In less than 1% of all cycles,the crusts are thick enough to require a full power action. This meansthat in a vast majority of the crust breaking cycles, the required airpressure behind the working piston is very low, as is the pressure airvolume fed into the actuator cylinder. The above described restrictedair feed to the actuator means a certain reduction in the consumedpressure air volume compared to previously used full pressure actuatoroperations, and of course it means a substantial cost saving for theindustry. A condition for this, however, is that the piston is allowedto return to its start position immediately after reaching its extendedextreme position, otherwise, there will still be a full pressurebuild-up in the actuator cylinder and a resulting pressure air waste.

[0006] Due to reasons as customer requirements and slow signalcommunication between position sensing means at the electrolytic pot anda control unit, the piston in previous actuators has been maintained forsome time in its extended end position, which means that even if you usefeed flow restrictions to keep down the drive pressure on the pistonduring piston movement, there will still be a full pressure build-up inthe actuator cylinder after the piston has completed its strokes. Suchpressure build-ups are of no use but a waste of expensive pressure air.

[0007] The main object of the present invention is to accomplish apneumatic actuator system by which the pressure air consumption isbrought down to a minimum such that no more pressure air than absolutelynecessary is spent on the actuator operation while automaticallyproviding maximum pressure and top power capacity when ever required.

[0008] Another object of the invention is to provide a pneumaticactuator system having short and quick air communication routes, so asto make the actuator operation distinct and without any delays inrelation to given command signals.

[0009] A further object of the invention is to enable operation of morethan one actuator by a single directional valve.

[0010] A still further object of the invention is to provide an actuatorsystem wherein components sensitive to harsh environmental factors likeheat, strong magnetic fields, chemically active substances etc. may belocated remotely from the actuator without increasing the pressure airconsumption.

[0011] Other objects and advantages of the invention will appear fromthe following specification containing a detailed description ofpreferred embodiments of the invention with reference to theaccompanying drawings.

[0012] In the drawings:

[0013]FIG. 1 illustrates schematically a section through an electrolyticbath in an aluminium producing plant, including a pneumatic actuator forcrust breaking purposes.

[0014]FIG. 2 shows schematically an actuator system according to oneembodiment of the invention.

[0015]FIG. 3 shows an actuator system according to an alternativeembodiment of the invention.

[0016]FIG. 4 shows an actuator system according to a second alternativeembodiment of the invention.

[0017] As mentioned above, the pneumatic actuator system according tothe invention is suitable for crust breaking operations in the aluminiumproducing industry. One type of aluminium producing plant comprises anumber of electrolytic pots, and in FIG. 1 there is shown one suchelectrolytic pot 10 containing an electrolytic bath 11 and having abottom cathode 12 and two anodes 13. The anodes 13 are movably supportedon an overhead structure 15 (not shown in detail), and a singlepneumatic actuator 14 mounted on the same structure 15. On top of theelectrolyte 11, there is inevitably formed a crust layer 16 comprisingresidual material from the alumina reduction process.

[0018] As an electrolytic reduction process is going on, a crust layeris continuously formed on top of the bath, and to be able to add morealumina to the bath during the process the crust layer has to berepeatedly broken. To this end, the pneumatic actuator 14 is mountedvertically and provided with a crust breaking working implement 17, andwhen it is decided to accomplish a hole in the crust layer 16, theactuator 14 is activated to force the working implement 17 right throughthe crust layer. For adding alumina to the bath there is provided a socalled point feeding device by which alumina is supplied right throughthe hole made by the working implement 17. The alumina feeding device isnot a part of the invention and is therefore not described in furtherdetail.

[0019] In FIG. 2 there is described an actuator system according oneembodiment of the invention which comprises a piston-cylinder typeactuator 14 having a cylinder 20, a piston 21 and a piston rod 22. Thelatter is intended to engage an external load of varying magnitude, forinstance via a crust breaking implement 17 as described above. Thesystem further comprises an actuator control circuit which includes adirectional valve 24 connected to a pressure air source 25 and which hasair communication ports for directing pressure air to and from theactuator 14. The directional valve 24 is spring biassed in one directionand pressure air activated by a start command signal in the oppositedirection. The start command signal is supplied via a conduit 23.Alternatively, the start command signal may be provided as an electricalsignal from a remote control unit for actuating an electromagnetic airvalve located close to the directional valve 24.

[0020] The directional valve 24 shown in FIG. 2 also comprises flowrestrictions 26,27 located in the alternative air feed passages throughwhich pressure air is supplied to the actuator14. Alternatively, theseflow restrictions may be replaced by a single restriction located at theinlet port of the directional valve 24. However, the purpose andfunctional features of the flow restrictions 26,27 will appear from thefollowing specification.

[0021] The control circuit further comprises two end position sensingvalves 28,29 which are built-in in the actuator cylinder 20 fordetecting and indicating whether the piston 21 has reached its extremeend positions.

[0022] Two air shut-off valves 30,31 are provided to alternatively letthrough or block air flow to and from the actuator 14, respectively,dependent on the current position of the piston 21 as detected by theend position sensing valves 28,29. Whereas the position sensing valves28,29 are mechanically activated by the piston 21, the air shut-offvalves 30,31 are pressure air activated. The position sensing valves28,29 are spring biassed towards their closed positions, whereas the airshut-off valves 30,31 are spring biassed towards their open positions.

[0023] In operation of the actuator system, the directional valve 24 isgiven a start command signal via the conduit 23, whereby the valve 24 isshifted against the spring bias force to establish communication via theflow restriction 26 between the pressure air source 25 and an aircommunication passage 34. Since the air shut-off valve 30 is in itsinactivated open position, there is free communication to the rear endof the cylinder 20, i.e. the driving side of the actuator piston 21. Atthe same time, however, the idling side of the piston 21, i.e. thepiston rod side, is prevented from being vented through conduit 35 inthat the shut-off valve 31 is closed. This is because the positionsensing valve 29 is activated by the piston 21 and supplies pressure airto the maneuver side of the shutoff valve 31. However, due to a largerpressurised area at the rear end of the piston than at the piston rodend, and due the vertical orientation of the actuator 14 and the totalweight of the piston 21, piston rod 22 and the working implement 17, acertain downward movement of the piston 21 will take place, long enoughto deactivate the valve 29 and stop pressurising the valve 31 to closedposition.

[0024] Now, the air shut-off valve 31 is shifted to its inactivatedspring maintained open position to duct away vented air from theactuator 14 through the communication passage 35 and the directionalvalve 24.Thereafter, the piston 21 is able to start moving downwards, tothe left in FIG. 2, so as to perform a crust breaking working stroke.

[0025] Due to the flow restriction 26 in the directional valve 24, theair feed to the actuator 14 takes place slowly, and since there is noflow restriction in the vent passage of the valve 24, the air on theidling side of the piston 21 will be vented to the atmospheresubstantially without any back pressure. The restricted air feed to theactuator 14 prevents pressure from being built-up on the driving side ofthe piston 21 to a higher level than what is actually needed for thepiston 21 to perform a working stroke and to reach its fully extendedposition. In case of a massive crust layer, a high pressure is requiredto move the piston, and as long as the end position sensing valve 28 isnot activated, pressure air is continuously fed into the actuatorcylinder 20 successively increasing the pressure until the piston 21eventually reaches its fully extended position and the end sensing valve28 is activated. When activated, the end sensing valve 28 opens upcommunication through the conduit 33 between the start signal conduit 23and the maneuver side of the shut-off valve 30 making the latter shiftto closed position. Thereby, the pressure air feed to the actuator 14 isstopped at once. An o.k. signal may be obtained via a conduit 37connected downstream of the end sensing valve 28. Such a signal may beused for remote control of the process.

[0026] The above described condition will prevail until the startcommand signal in conduit 23 is discontinued. The actuator piston 21remains in its fully extended position, and no further pressure air issupplied to the driving side of the piston 21.

[0027] When the start command signal in conduit 23 is discontinued, thedirectional valve 24 returns by spring force to its original position,to the left in FIG. 2, wherein instead the pressure air source 25 isconnected to the piston rod side of the actuator piston 21 via passage35. This communication is open since the end position sensing valve 29occupies its inactive closed position, and the air shut-off valve 31occupies its spring maintained open position. Venting of the rear idlingside of the piston 21 is established in that the pressure of the startcommand signal supplied via conduit 33 and the activated valve 28 stopsacting on the maneuver side of the shut-off valve 30 making the latterreturn to its inactive open position.

[0028] Now, the piston 21 starts moving upwards, to the right in FIG. 2,and because of the air feed restriction 27 in the directional valve 24,no more pressure air is supplied to the actuator than what is needed tolift the piston 21, piston rod 22 and working implement 17 back to theirupper rest positions. The upper or right hand side of the piston 21 isvented through passage 34. As soon as the piston 21 reaches its fullyretracted position, the end sensing valve 29 is shifted to its openposition, against a spring bias force. Thereby, communication isestablished between the maneuver side of the shut-off valve 31 and thepressure air source 25 via a passage 38, resulting in a shifting of theshut-off valve 31 to its closed position, as illustrated in FIG. 2. Asin the opposite end position, an o.k. signal may be obtained via conduit39 connected downstream of the end position sensing valve 29.

[0029] From the above description of the actuator system it is apparentthat by the employment of the air shut-off valves 30,31 and the endposition sensing valves 28,29 there is obtained an instantaneouspressure air shut-off as the piston 21 reaches either one of its extremeend positions. Whereas the directional valve 24 normally has to belocated at a distance from the actuator 14 and the harsh environment inthe close vicinity of the electrolytic bath, the shut-off valves 28,29which are of a simple and rugged design may be located close to theactuator 14 so as to accomplish a very quick and distinct air shut-offwithout any unnecessary delays. The combination of end position sensingvalves and separate air shut-off valves provides a substantiallyimproved pressure air economy, because the needed air pressure and theconsumed air volume are continuously and automatically kept at a minimumlevel.

[0030] In FIG. 3, there is illustrated an alternative embodiment of theinvention, wherein air feed flow restrictions 26 a,27 a are integratedin the air shut-off valves 30 a,31 a. This means a further improvementof the actuator control function, because in this case the pressuredrops caused by the long conduits between the directional valve 24 andthe actuator 14 are minimized since a less sensitive full pressure airfeed is maintained all the way up to the shutoff valves 30 a,31 a. Inorder to avoid flow restrictions on the vented side of the actuatorpiston 21, the shut-off valves 30,31 have been provided with shunts40,41 including check valves 42,43.

[0031] By the location of the air feed restrictions 26 a,27 a to theshut-off valves 30 a,31 a, it is made possible to obtain pressure airsupply to the position sensing valves 28,29 via conduits 33 a,38 aconnected to the conduits 34,35 where full pressure is available whenrequired. So, air supply conduits 33 a and 38 a may be connected to theconduits 34,35 at a location close to the actuator 14 instead of alocation close to the directional valve 24. This reduces the number ofconduits between the directional valve 24 and the actuator 14. It alsomeans that the directional valve 24 can be located at a distance fromthe actuator 14 away from the aggressive atmosphere around theelectrolytic bath. A further advantage gained by this alternativelocation of the air feed restrictions 26 a,27 a is a less complicateddirectional valve 24, i.e. the directional valve 24 may be of a simpleconventional design.

[0032] A slight variation of the above described device is illustratedin FIG. 4. Instead of having a spring biassed directional valve 24 whichautomatically returns to its operation start position as soon as thestart command signal is discontinued, there is employed a bi-stabledirectional valve 24 a. An OR-gate 36 is connected between the o.k.signal conduit 37 and one maneuver side of the directional valve 24 a.By this OR-gate 36 it is possible to reset the directional valve 24 aeither automatically by the o.k. signal obtained from the end positionsensing valve 28 or by a reset signal provided by a remote control unit(not shown).

[0033] It is to be noted that the embodiments of the invention are notlimited to the described examples but may be freely varied within thescope of the claims.

[0034] For instance, the actuator system according to the invention maybe used at alumina reduction pots where the crust layer breaking devicecomprises a horizontal crust breaking beam. In that application, oneactuator is connected at each end of the breaking beam for vertical,substantially parallel movement of the beam through the crust layer. Thetwo actuators are fed with pressure air by a common directional valve,and the flow restrictions in the feed passages of the directional valvewill be effective in distributing the air flow to both actuators inresponse to their individual instant load, such that the actuator havingthe lowest load gets the most pressure air. This means that the drivepressures in the actuators. are automatically adapted to the actualindividual load level, such. that when one of the actuators has reachedits extreme positions and the other has not the latter will becontinuously pressurised until it has reached its extreme end positionas well. Meanwhile, the air supply to the first actuator to reach itsextreme end position is cut off by the respective air shut-off valve.

1. Pneumatic actuator system, comprising: one or more piston-cylindertype actuators (14) each having a working piston (21) with a loadengaging piston rod (22), a control circuit including a directionalvalve (24;24 a) connected to a pressure air source (25) and arranged todirect pressure air to alternative driving sides of the working piston(21) of each actuator (14) for accomplishing movement of the workingpiston (21) in alternative directions, and an air flow restricting means(26,27;26 a,27 a) connected to the actuator (14) for restricting the airfeed flow to the current driving side of the working piston (21),characterized in that each actuator (14) is provided with end positionsensors (28,29) for detecting and indicating the extreme end positionsof the working piston (21), and air feed shut-off valves (30,31; 30 a,31a) connected to said end position sensors (28,29) and arranged to cutoff the air feed to the current driving side of the working piston (21)as an extreme end position is reached and indicated by the respectiveend position sensor (28,29).
 2. Actuator system according to claim 1,wherein said directional valve (24;24 a) is located remotely from theactuator or actuators (14), whereas said air feed shut-off valves(30,31; 30 a,31 a) form a unit together with the respective actuator(14).
 3. Actuator according to claim 2, wherein said air flowrestrictions (26 a,27 a) are located in said air feed shut-off valves(30 a,31 a).
 4. Actuator according to claim 2, wherein said shut-offvalves (30,31; 30 a,31 a) are mounted on the outside of the respectiveactuator (14), whereas said end position sensors (28,29) are built-in inthe respective actuator (14).
 5. Pneumatic actuator system for crustbreaking in electrolytic aluminium reduction baths (10), comprising oneor more vertically oriented piston-cylinder actuators (14) each having aworking piston (21) with a piston rod (22) connected to a crust breakingimplement (17), a control circuit including a directional valve (24;24a), and flow restrictions (26,27) inserted between the actuator (14) andthe directional valve (24;24) for restricting the air feed flow to thecurrent driving side of the working piston (21), characterized in thateach actuator (14) is provided with end position sensors (28,29) fordetecting and indicating the extreme end positions of the working piston(21), and air feed shut-off valves (30,31; 30 a,31 a) connected to saidend position sensors (28,29) and arranged to cut off the pressure feedto the current driving side of the working piston (21) as an extreme endposition of the working piston (21) is reached and indicated by therespective end position sensor (28,29), said end position sensors(28,29) and said air feed shut-off valves (30,31; 30 a,31 a) aredisposed integrally with the actuator (14) to form a working unit to belocated at the electrolytic reduction bath (10), whereas saiddirectional valve (24,24 a) is located remotely from the actuator (14).6. Actuator system according to claim 5, wherein said flow restrictions(26 a,27 a) are integrated with the air feed shut-off valves (30 a,31a).
 7. Actuator system according to claim 5 or 6, wherein two actuators(14) have their working pistons connected to a common substantiallyhorizontally oriented crust breaking beam, said actuators (14) sharing acommon remotely located air feeding directional valve (24;24 a) butcomprise separate end sensors (28,29) and air feed shut-off valves(30,31; 30 a,31 a).
 8. Actuator system according to claim 5 or 6,wherein each actuator (14) operates a single-point crust breakingworking implement (17) which extends in a substantial co-axialdisposition relative to said piston rod (22).